1. Field of the Invention
The present invention relates generally to the field of preamble formats for multi input multi output (MIMO) systems and particularly to a method and apparatus for merging the benefit of the Greenfield preamble format into a mixed mode preamble format with forward/backward compatibility of 802.11a, 802.11g and 802.11n standards.
2. Description of the Prior Art
Wireless Local Area Network (WLAN) is popular technology for home, office and public wireless access of the Internet. The WLAN industry has become so successful that new applications rapidly continue to evolve. Example applications like handset, Personal Digital Assistant (PDA), medium adaptor, and video streaming. The expanding demand returns to drive the further progress of WLAN industry. IEEE 802.11n, a standard adopted by the industry, sets forth new WLAN standards to respond to this need.
Multiple-input-multiple-output (MIMO) is prevalently employed in communication equipment and devices. MIMO-OFDM (Orthogonal Frequency Division Modulation) is becoming the modulation scheme of the new high speed WLAN standard, the IEEE 802.11n. MIMO-OFDM systems include multiple transmitters and receivers for transmitting and receiving streams of data using OFDM. MIMO-OFDM systems can be denoted as nTmSrR, where ‘n’ is the number of transmitter antennas, ‘m’ is the number of data streams and ‘r’ is the number of receiver antennas. The new WLAN standard is expected to support high throughput and extended range through MIMO-OFDM.
WLAN is based on medium sharing technology, which is packet-based instead of using direct link. Packets provide excellent framework for sharing. For every transmitted packet, there is a preamble associated therewith. A preamble is used to train and initialize receivers before sending data that follows the preamble. To achieve high throughput, the overhead associated with the preamble length is critical for high PHY rate. Efficiency becomes one important criterion for preamble design.
Given the broad scope of the new WLAN standard, many MIMO-OFDM schemes need to work under the same framework, therefore besides efficiency, interoperability with legacy WLAN devices and forward/backward compatibility within 802.11n modes become another challenge for preamble design.
Previous works on MIMO preamble are showed in proposals from 80211.n standard subgroups, such as WWiSE and TGnSync group. The major drawback of TGnSync preamble design is lack of efficiency, which impacts throughput especially when a packet length is short. The shortcomings of WWiSE preamble design are lack of flexibility to accommodate different features, such as transmit beamforming, in addition to less well thought on interoperability issue between supported 11n modes.
In light of the foregoing, what is needed is a framework combining the strength of both WWiSE and TGnSync preambles.